java 16 lines · 5 steps

Building maps from lists with Collectors.toMap

Two stream pipelines turn a list of customers into id-keyed maps, one handling duplicate keys and ordering explicitly.

Explained by highlit

Intermediate streams collectors maps functional

1public Map<Long, CustomerDto> indexByCustomerId(List<CustomerDto> customers) {
2    return customers.stream()
3            .collect(Collectors.toMap(
4                    CustomerDto::getId,
5                    Function.identity(),
6                    (existing, replacement) -> replacement,
7                    LinkedHashMap::new));
8}
9 
10public Map<Long, String> emailByCustomerId(List<CustomerDto> customers) {
11    return customers.stream()
12            .filter(c -> c.getEmail() != null)
13            .collect(Collectors.toMap(
14                    CustomerDto::getId,
15                    CustomerDto::getEmail));
16}

01 / 01

STEP 01

Walkthrough

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Three takeaways

1The two-argument `toMap` throws on duplicate keys, so supply a merge function whenever collisions are possible.
2`Function.identity()` keeps the whole element as the value, letting you index a list by any of its fields.
3Passing a map supplier like `LinkedHashMap::new` gives you control over iteration order and map type.

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